1. Field of the Invention
The present invention relates to a lockup device of a hydraulic torque transmission device, and particularly to a lockup device having a plurality of friction surfaces.
2. Background Information
A torque converter has been known as a hydraulic torque transmission device. A torque converter generally has three kinds of internal vane wheels (an impeller, a turbine, and a stator) for transmitting torque via an internal working fluid. The torque converter is provided with a lockup device in many cases.
The lockup device is arranged in a fluid chamber formed by a turbine and a front cover, and particularly in a space between the turbine and the front cover, and is a mechanism for directly transmitting the torque from the front cover to the turbine by mechanically coupling the front cover to the turbine.
Generally, the lockup device is formed of a circular plate-like or disk-like piston that can be pressed against the front cover, a friction plate to be held between the piston and the front cover, and a damper mechanism that can elastically couple the front cover to the turbine in a rotating direction via the friction plate, e.g., Japanese Laid-Open Patent Publication H10-47453.
When the lockup device is in an engaged state, hydraulic pressure moves the piston toward the front cover to hold the friction plate between the piston and the front cover. Consequently, the torque is transmitted from front cover to the friction plate, and further, is transmitted to the turbine via the damper mechanism.
When the lockup device is in a disengaged state, the hydraulic pressure moves the piston toward the turbine to allow the friction plate to rotate relatively to the front cover and the piston. Consequently, the torque is transmitted from the impeller to the turbine via the fluid without passing through the lockup device.
However, a drag torque may occur during the disengaged state of the lockup device. More specifically, when it is in the disengaged state, the fluid flows radially outward through a space between the friction plate and the front cover or between the friction plate and the piston. Thereby, the friction plate is pulled toward the front cover or the piston. Consequently, a slide resistance occurs between the friction plate and the front cover or between the front cover and the piston in spite of the fact that the device is in the disengaged state, and this slide resistance causes the drag torque.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved lockup device and hydraulic torque transmission with the lockup device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.